Field of the Invention
The present invention relates to floating lift-driven wind turbines, and more particularly, to a floating hybrid vertical/horizontal axis wind turbine designed to assume a predetermined heel angle that can be chosen to maximize the electrical power generated by the wind at a specific offshore location with known environmental conditions and to a floating hybrid wind turbine that generates nearly constant power over a range of heel angles.
Description of Related Art
The power industry is increasingly developing ways of generating electricity other than fossil fuels and nuclear energy. Many sources of renewable energy are being considered, but wind is one of the most popular. The number of “wind farms,” sometimes comprising hundreds of wind turbines arrayed over a several square miles, is steadily increasing. However, the only type of wind turbine currently used for large, utility-scale power generation is the horizontal axis wind turbine (HAWT). Vertical axis wind turbines (VAWT) are a known alternative t HAWTs, and the applicant's above-referenced U.S. Pat. No. 10,208,734 discusses at length the advantages, and disadvantages, of heretofore known types of lift-driven VAWTs versus HAWTs.
For a variety of reasons, the industry has focused more on improving HAWT technology than on developing VAWTs. As pointed out in reference U.S. Pat. No. 10,208,734, utility-scale VAWT technology is not as mature as that for HAWTs, with no known VAWT systems currently being offered or produced by existing utility-scale turbine manufacturers. VAWTs produced in the past that were considered utility-scale at the time are too small to be considered as such (by a factor of ten or more) by current standards. But the focus has been shifting to VAWTs for deep-water offshore applications where the turbine will be mounted on a floating platform. One reason is the argument that taking into account factors such as operation and maintenance costs, capital investment, and other expenses involved in generating electricity, VAWTs' total cost of energy (COE) has the potential to be competitive with, or even lower than, COEs for HAWT designs. See, for example, Paquette, Joshua, et al., “innovative Offshore Vertical-Axis Wind Turbine Rotor Project,” Proc. of European Wind Energy Assoc., Copenhagen, Denmark, Apr. 16-19, 2012.
However, to become commercially viable VAWTs will still have to be made at utility-scale sizes without adversely affecting their potential COE advantage. This is because according to principles of physics, the size of a turbine determines the total power PW it can extract from the wind, expressed by the formula:
                              P          W                =                              {                                          (                                  1                  2                                )                            ⁢              ρ              ⁢                                                          ⁢                              U                3                            ⁢              A                        }                    ⁢          Cp                                    (        1        )            where ρ=density of air, U=wind velocity, A=projected area of the turbine (as defined further below), and Cp is the power coefficient, which is a measure of turbine efficiency. As seen by this equation, PW can be increased by making the turbine larger (increasing the value of A in eq. 1). But increasing the size (projected area A) of the turbine requires not only that the principal turbine parts—blades and struts—be longer, but also that they be made more robust. If all three linear dimensions of the blades and struts must be increased, then the weight and the cost of materials increase as the cube of the size (for example, doubling each linear dimension would increase cost by a factor of eight), which results in an increase in overall COE. The unique wind turbine described in reference U.S. Pat. No. 10,208,734 represents a significant advance toward making larger size VAWT-type turbines commercially viable through a design that lowers the cost-of-material contribution to COE by reducing the loads on the larger turbine structure that would otherwise require more robust blades and struts.
Even as technical advances make it possible to scale up VAWTs, it is anticipated that the present level of resistance to land-based wind farms will continue to increase. National Wind Watch, Inc. (www.wind-watch.org), cites a long list of organizations around the world opposed to the use of wind as a renewable energy resource (see also North American Platform Against Wind Power, www.na-paw.org). While some observers have expressed doubt about the technical efficacy of using wind power in any manner, many of the arguments against it relate either to adverse effects of land-based HAWTs on the environment (unsightly appearance, danger to birds, etc.), or on those living in proximity to them (noise, perceived ambient pressure fluctuations, etc.). The FAA and the U.S. military have also expressed concerns about locating HAWT-based wind farms near aviation sites. “Wind Turbine Projects Run Into Resistance,” New York Times, Aug. 26, 2010 (“In 2009, about 9,000 megawatts of proposed wind projects were abandoned or delayed because of radar concerns raised by the military and the Federal Aviation Administration, according to a member survey by the American Wind Energy Association. That is nearly as much as the amount of wind capacity that was actually built in the same year, the trade group says.”) (http://www.nytimes.com/2010/08/27/business/energy-environment/27radar.html).
As a result, the interest in developing offshore wind farms of any type, whether they comprise HAWTs or VAWTs, has intensified in recent years. In fact, offshore wind farms have long been a favored approach because oceans, seas, and large lakes have plentiful locations where prevailing winds are more reliably and constantly higher than over land. HAWTs are not particularly suited to deep-water installations for well known reasons, some of which are discussed in reference U.S. Pat. No. 10,208,734. Most of these reasons are inherent to the HAWT configuration, where the heavy turbine rotor and its associated power generation equipment are located hundreds of feet above the surface and the tower supporting them must remain vertical. Presently, the industry has not determined a practical way around the necessity of locating an HAWT in water shallow enough to secure its supporting platform in the seabed, which means in almost all cases that offshore HAWT wind farms will still be in sight of land, at least for the foreseeable future. To be out of sight of an observer on land, a 200 m tall wind turbine would have to be roughly 35 miles offshore. Most locations in the large bodies of water of the world are too deep to justify the increase in COE resulting from the cost of infrastructure required to secure the HAWT to the underlying seabed.
Another reason for increasing interest in VAWTs is that their configuration makes them much more suitable for floating offshore installation because their low center of gravity makes it easier to stabilize them in the presence of prevailing winds, as discussed in the referenced application. VAWTs also do not have to be oriented at a specific angle to the oncoming wind—a significant advantage over HAWTs, which require costly equipment for turning them into the wind. The design approach for free-floating installations of VAWTs has heretofore been to seek ways to limit the amount they heel at an angle to vertical in the presence of a prevailing wind. See, for example, Paquette, Joshua, et al., which discusses ways to stabilize floating VAWT platforms. In Europe, Nenuphar, S. A., of Lille, France, has been working since 2006 on bringing floating VAWTs to commercial application. Its approach is to limit the amount by which the VAWT is permitted to heel so that the platform can be made smaller. “Vertiwind: Making Floating Wind Turbine Technology Competitive for Offshore,” Nenuphar, S. A., October 2012 (http://www.twenties-project.eu/system/files/2_2013-03%20Presentation %20short.pdf). More recently, Nenuphar has proposed using counter-rotating VAWTs on the same platform, which it claims will reduce heeling moment by at least 40%, and in turn reduce platform cost. “The Nenuphar Solution—Nenuphar Wind,” Nenuphar, S. A., 2015 (http://www.nenuphar-wind.com/en/15-the-nenuphar-solution.html). While this approach may work once it is tried on an actual installation, the physics of using wind energy to generate electricity will probably drive development toward larger and larger VAWTs to increase their capacity. As VAWTs are made larger, the heeling moment will perforce increase to the point where platform constructions and gearing arrangements for multiple VAWTs on a single platform may result in unacceptable COE increases.
What is needed is a fundamentally new design paradigm that will still enable VAWTs to be scaled up to sizes that can make a meaningful reduction in their CUE (per referenced U.S. Pat. No. 10,208,734), and will also allow offshore installation at distances that place them over the horizon, while avoiding complex and expensive arrangements (like counter-rotating VAWTs mounted on the same platform).